ospf multizone understanding

[Rowlands Sharles]

Hello support,

I have a question regarding ospf with multiple areas, it's possible to have a area dehind another area outside zone 0?

Attached is the diagram, can we have area 10 ?

Thanks

[MHM Cisco World · Accepted solution]

Yes you can use virtual link

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47866-ospfdb7.html

MHM

[Rowlands Sharles]

Okay, that means if you use this scenario, you must therefore use the virtual link?

[MHM Cisco World · Accepted solution]

Yes You need it since area 10 can not direct connect to backbone area 0

MHM

[Rowlands Sharles]

Okay, many thanks, will read the document sent.

[Rowlands Sharles]

Hello,

Last thing please, how can i know the number of routers in an area?

[Robert Plunkett · Accepted solution]

Remember, when creating your virtual link you should use you ospf router ID.

[Torbjørn]

@MHM Cisco World is correct that a virtual link can be used to achieve this. I just want to add that you should avoid this situation if possible. If you do find yourself in a situation where you have to use a virtual link, you should only rely on this as a temporary fix.

[paul driver · Accepted solution]

Hello
based on your topology-why not have that rtr in the same area so no need for a VL?

[Rowlands Sharles]

Hi Paul driver

you proposition is correct. In my case, i have manys sites connected one after one by WAN links (each site is a different city).

So, to separate things, i think it's better to put each site on a separate area.

Regards

[Joseph W. Doherty · Accepted solution]

Possibly overkill, but I would consider insuring each area has a physical attachment to area zero.

If you do that, you have two choices, which are, extend site's area up to HQ ABR, or make site WAN router an ABR.  There are pluses and minuses to both approaches, also impacted by advanced Cisco OSPF configuration options.

[Joseph W. Doherty · Accepted solution]

Is it possible, yes.  Will it work correctly, not without some additional configuration.

Others have explained using virtual link or not having the separate remote area 

Other possible options would be to use tunnels and/or a separate VLAN, to either extend the remote area to an ABR, or extend area zero to that area.

[imhessam]

All non-backbone area's in OSPF need to be connected to the backbone area (database exchange). as others explained you can use virtual link considering it's a temporary solotion (DNA flag on LSA, dependency on transit area, ......)